The present invention relates to sealed combustion furnaces-- that is, furnaces in which the combustion air is taken from the atmosphere rather from the room or space being heated. More particularly, the present invention relates to a horizontal vent air terminal for sealed combustion furnaces.
The term "air terminal" refers to the terminal portion of the flue pipe and the location at which combustion air is drawn from the atmosphere. Since both conduits must pass through a vertical wall or a horizontal vent, it is desirable, to minimize installation costs, that they both pass through the same hole.
In furnaces of this type, it is necessary to achieve and maintain a properly balanced flow of combustion air into the system and flue products out of the system, while preventing the mixing and recirculation of flue products with fresh combustion air. In designing vent terminals capable of performing under incident wind conditions, prior practice has been to counteract or offset the effect of the wind in an attempt to simulate static operating conditions to maintain the critical balanced flow of intake air and combustion products. Previous test and industry-approved requirements for systems of this type did not require the maintenance of proper combustion characteristics (that is, requirements on carbon monoxide and carbon dioxide in the flue gases), under varying conditions. Rather, prior requirements were directed primarily to maintaining flame stability (that is, to prevent main burner flame or pilot flame outages) under varying wind conditions. Even with these requirements, prior designs of vent terminals for sealed combustion furnaces on recreational vehicles have proven suscepticle to standing pilot and main burner outages under various wind conditions; and this has been a continuing problem through the years, although in various degrees, as winds upset the critical system balance. The most common problem with vent terminals of this type is that an incident wind causes exhaust gases to splatter against the wall in all directions. Some of the splattered gas, therefore, must recirculate.
Another problem sometimes experienced in prior designs for sealed combustion systems, whether having an induced or a forced draft is that a standing pilot has been more susceptible to outage than in sealed combustion systems with a natural draft, for example, when venting through a roof. One of the reasons, of course, is that there is much less space, and hence, much less oxygen, available in an induced draft system than in a natural draft system. Such a pilot outage has occurred generally when the main burner is off so that the induced draft is not in operation, and the operation of the pilot is dependent primarily upon its ability to generate enough heat to create a natural draft and maintain a stable flame. During extremely cold weather, the ability of the pilot to generate sufficient heat to maintain a natural draft through a horizontal vent is, of course, reduced.
The reduced velocity of the pilot gases through a horizontal flue is not normally sufficient to cause pilot outage if the external vent terminal is not subjected to an incident wind. However, when the incident wind velocity exceeds 40 miles per hour or is in the range of below 10 miles per hour and the main burner is not operating, pilot outage may be caused by a blockage of the vent terminal, thereby suffocating the pilot as oxygen is consumed in the small burner cavity, or by recirculating flue products, further aggravating the problem of not having enough oxygen available to maintain a stable pilot.
The present invention represents an improved air terminal over that shown in the Honaker, U.S. Pat. No. 3,643,646, issued Feb. 22, 1972, entitled "Flue Exhaust and Combination Air Intake Assembly for Undercounter Furnace", and co-owned herewith. The stages of development will be more fully explained within for a better understanding of the structure and operation of each combination. However, previous air terminals exhibited certain disadvantages.
The previous air terminal which was actually manufactured (sometimes referred to as the "commercial" prior art) was somewhat different than the air terminal disclosed in the above-identified patent (sometimes referred to as the "patented" air terminal). Briefly, the patented air terminal employed a cone-shaped "frustum" located in front of the exhaust outlet of the flue pipe for creating turbulence in the exhaust gases after they were discharged; but this lack of directional flow of the exhaust gases became accentuated during high incident winds, resulting in a splattering of the gas and a recirculating of a portion of the exhaust gases. The amount of recirculation increased with incident wind velocity.
The commercial air vent improved the operation by eliminating the cone-shaped frustum and by extending the exhaust flue to a location approximately co-planar with the outer surface of the exterior wall through which the vent extended. This improved operation somewhat, in terms of reducing recirculation of flue gas, but, as will be explained within, the present invention is a significant improvement even over the commercial embodiment of the previous air terminal.
Briefly, the present invention provides an air terminal for a horizontal vent of a sealed combustion furnace which includes a faceplate mounted on the outside of an exterior vertical wall. A horizontal flue extension receives combustion products from the furnace and exhausts them to the atmosphere. An oval sleeve surrounds the flue extension, and fresh air is communicated to the furnace through the space between the flue pipe and the sleeve.
The outlet of the flue pipe is approximately 3/4 in. beyond the faceplate or approximately 1 in. beyond the surface of the wall. The longer dimension of the oval sleeve extends vertically, and the flue pipe is located in the upper portion of it, held by tabs connecting the pipe to the air terminal, and by a horizontal wind deflector vane. Otherwise the fresh air intake opening conforms to the shape of the sleeve and in unobstructed.
A pocket-shaped rain shield is located beneath the flue pipe, and it extends upwardly and inwardly. The wind vane extends across the oval sleeve at approximately its midpoint and in front of the upper extension of the rain shield. The wind vane is designed to deflect incident wind downwardly in a substantially uniform manner parallel to the wall; and during high incident winds, it cooperates with the rain shield to deflect air downwardly along the lower portion of the fresh air intake opening and thence downwardly along the exterior wall.
For incident wind in the range up to 5 m.p.h., the operation of the inventive system is the same as for no wind--namely, the flue gases are delivered at a location spaced outwardly of the wall and travel upwardly due to their elevated temperature. The fresh combustion air is drawn through the aperture in the terminal plate beneath the wind vane, are deflected upwardly by the rain shield, and are delivered to the furnace through the space between the sleeve and the flue pipe. Somewhere in the range of 5-10 m.p.h. for incident wind, the operation of the system changes dramatically, as shown in smoke tests. That is, the exhaust gases are forced downwardly by the draft created by the wind vane, and fresh combustion air is taken in through the enlarged opening in the upper portion of the terminal plate.
As mentioned, this mode of operation for the improved air terminal of the present invention begins for incident wind velocities in the range of about 5-10 m.p.h., and it continues up to velocities over 40 miles per hour. The air flow is the same whether the furnace operates on pilot or on burner (that is, full burner operation) modes.
With the present invention, significant improvements have been observed in reducing the amount of carbon monoxide in the flue gas for all wind speeds. The improvement is particularly noticeable in relation to the prior commercial version of air terminal for wind speeds greater than about 5 m.p.h. Further, the present invention significantly controls the amount of carbon dioxide at a desirable efficiency level in the flue gas for all wind ranges, again, the improvement relative to the prior commercial version increasing with increased wind velocity.
Thus, the present invention provides significant advantages in maintaining acceptable levels of both carbon monoxide and carbon dioxide in the flue gas under main burner operation, and the improvement increases as the wind velocity increases. Further, because of the natural draft created in the system by incident wind, pilot outage has also been significantly reduced.
Other features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment accompanied by the attached drawing.